A typical radio frequency identification (RFID) tag has a memory configured to store data, such as data that uniquely identifies the tag among a plurality of RFID tags. An RFID reader can be used to communicate with RFID tags over a radio link. Thus, the identity of the tag and the object labeled by the tag can be determined by the RFID reader in an automated process.
In a typical RFID system, an RFID reader is configured to interrogate the tags via radio frequency electromagnetic waves. The RFID reader broadcasts commands using a beam of electromagnetic wave. In response to the interrogation signals from the RFID reader, an RFID tag can be selected to produce a radio frequency response signal.
An RFID tag may be passive, semi-passive or active, depending on the power source of the RFID tag and how the response from the RFID is generated.
A passive RFID tag does not have an internal battery or power source. A passive RFID tag operates using the power drawn from the interrogating electromagnetic wave. A passive RFID tag provides responses through modulating the interrogating electromagnetic wave backscattered by the tag.
A semi-active RFID tag operates using the power drawn from its internal battery or power source. A semi-active RFID tag provides responses through modulating the interrogating electromagnetic wave backscattered by the tag.
An active tag that has an internal battery or power source, using which a separate transmission signal is generated to provide the response. The response signal is generated independent from the interrogating electromagnetic wave.
Radio frequency identification (RFID) tags are used in a variety of applications, such as tagging vehicles on toll roads, tagging shipping containers, quality control on assembly line conveyor belts, and monitoring tactical military equipment maneuvers, etc.
Doppler effect is the change in the observed frequency of a wave, as perceived by an observer, when the source of the wave is moving relative to the observer. The frequency observed by the observer increases if the source of the wave moves towards the observer; and the observed frequency decreases if the source of the wave moves away from the observer. The Doppler effect has been used in some forms of radar to measure the velocity of objects.
In a classical example of a stationary observer listening to the whistle of a train moving relative to the listener, the pitch (frequency) of the whistle as perceived by the listener is higher as the train is approaching; and the pitch (frequency) of the whistle appears lower as the train is receding. In such a classical example, the source of the wave frequency is generated at the moving object (e.g., train).